Find a craigslist in a nearby city that contains several different universities and colleges. If you are unfamiliar with craigslist you can search for near by cities at http://geo.craigslist.org/iso/us.
At the bottom of the home page there will be a section titled "Gigs" with a subsection titled "Writing." Click on the writing category and select to post a gig. Select "I want to hire someone."
Offer only the bare amount of information. State the type of book you are working on (Romance, Science Fiction, Action, etc.), the fact you are preparing it for publication, and that you unable to pay for the service. Let them know you are hoping there are some journalism or English students that might be interested in editing the book so they can add it to their resume. Do not list your name, phone number, or any contact information. There will be a location where you can select to anonymize your email address.
The first time I did this, I received ten responses in 48 hours . . . and the responses kept coming in. Respond by sending them one chapter from your book (the same chapter to each editor) and a note suggesting they look it over. That way they will be able to see what and how you write, as well as what may need to be done. If they still want to edit your work, tell them to edit the chapter and send it back.
You do not have to tell them you definitely will go with them, or even that you have other options. Keep those options open as long as possible.
With your first response, make sure to provide no personal information other than your first name or email address. You can even open a generic yahoo or hotmail e-mail account just to sift through your responses.
When you get chapters back from the editors take time to look over their styles and compare what the different people found and how they phrased their changes. Writing is hard enough, you don't have to work with someone who makes you feel bad about what you are doing.
When you find someone you really like, send them more work.
I actually like to keep two or three of these free editors on tab. After one supplied edits and I do them, the chapter really should be reviewed again. Then if I submit the chapter to a critique group and they have suggestions, which I implement, it needs to be edited again.
If you don't get a good fit or would like to pick up additional editors, try other cities on criaigslist.